What is the likely hood of death for these mechanics?

Alright so, my party is fighting Vecna. I’m using the Vecna statblock from Eve of Ruin but a little nerfed (I nerfed the damage from both his Rotten Fate and the heal from his teleport ability as well as giving it a save), but I also added a ton of legendary actions as well as making this a multi phase fight. In classic dm fashion I waited till the last minute and did not have time to test these. So naturally my first instinct was to beg for help on here.

My party is level 15, with some very powerful magic items (Sword of Kas, Blackstaff, and a few others) and a few powerful homebrew magical abilities.

I also gave them a way to interrupt the Flight of the Damned and Rotten Fate actions from Vecna.

In Phase 3 I also removed the Flight of the Damned and Rotten Fate abilities but gave him the ability to use multiple spells in a turn.

TL;DR of the story is Vecna is almost a god and party has to stop him. Also this fight is in space (I can explain I swear).

Now, without further ado. Here are the legendary actions….Im sorry in advance.

Vecna, The Archlich (Phase 1)
Legendary Actions

Magic Atonement

Vecna curses a creature within 300ft to be unable to use magic without first atoning. The targeted creature must make a DC 22 Wisdom Saving Throw to be unaffected by this curse, otherwise the creature must sacrifice an additional spellslot of the same level to cast a spell, or sacrifice 10 x the spell’s level in HP in order to cast the spell. A creature can reroll this saving throw at the end of their turn. This curse lasts for one minute, or until the creature rolls a successful saving throw.

Relive Misery

Vecna warps a creature’s mind within 120ft, making them relive a painful memory. The targeted creature must make a DC 22 Intelligence Saving Throw or be stunned until the end of their next turn. 

Envelop

A Cloaker envelopes him, shielding him from all attacks with his body until the start of Vecna’s next turn. If the Cloaker survives the onslaught it joins the initiative and fights alongside Vecna.

The God of Nothing (Phase 2)
Legendary Actions

Soul Nail

Vecna attempts to tether the essence of creatures within 60ft of him to their current location. Creatures must make a DC22 Dexterity Saving Throw or become grappled until the start of Vecna’s next turn. 

Consume Soul

Vecna teleports to any creature of his choice affected by Soul Nail and attempts to consume their essence. Vecna makes a melee spell attack that deals 12d6+ Vecna’s Intelligence Necrotic damage, and heals Vecna for that same amount. Creatures are then freed from the Soul Nail effect.

Harrow

Vecna summons a Shadowspawn creature, as if summoned at 5th level, to pursue any creature not currently affected by Soul Nail.

The Will of Vecna (Phase 3)
Legendary Actions

Endless Death

Vecna assaults creatures within a 120ft cone, creatures caught in this cone must make a DC 22 Dexterity Saving Throw or take 12d6 Necrotic damage, halved on a success. Creatures who fail their saving throw have their movement speed halved (rounded down) until the start of their next turn.

Hurl Through Reality

Vecna attempts to throw a creature through the fabric of reality itself. A creature must make a DC 22 Wisdom Saving Throw or take 12d6 Psychic damage and be sent 30ft in a direction of Vecna’ choosing and become frightened until the end of their turn. Creatures that succeed their save take half damage and are not frightened or moved.

Partial Reconstruction

Vecna reconstructs a skeletal arm or a floating eye, granting him access to his Flight of the Damned or Rotten Fate ability. These body parts have 40 Hit Points and an AC of 16.